When entering, value is incorrect

Question

When checking the mounted Insert PHP with the form information, I can understand the value of the field data_final is not captured correctly, I would like to know what is wrong, because I believe it is a matter of formats.

Thanks in advance for the suggestions.

function that results in the data_final:

function calcula(){ 
      var periodo  = document.getElementsByName("periodo")[0].value;
      var dt1      = document.getElementsByName("datahora_inicial")[0].value;
      var val_t    = document.getElementsByName("f")[0].value;
      var parametros = {
              method: "GET"
      };

      fetch("php/calcula.php?periodo=" + periodo + "&dt1=" + dt1 + "&val_t=" + val_t, parametros).then(function(resposta) {
          return resposta.json();
      }).then(function(retorno){
          console.log(retorno)
          document.getElementById("dtfim").value = retorno;                         
      });
  }

<label class="col-sm-2 col-sm-2 control-label">Fim</label>//campo data 
  <div class="col-md-5">
    <input type="text" class="form-control round-input" name="datahora_final" required="required" id="dtfim" onchange="verifica();" value="" disabled="disabled"> 
 </div> 

php insert.

$datahora_final   = $_POST['datahora_final'];
$datahora_final   = date('Y-m-d H:i:s', $datahora_final);
$sql               = "INSERT INTO tabela(data_fim) VALUES ('$datahora_final')";

//Ao inserir, sql da data retorna este valor:
'1970-01-01 01:00:00'
e o seguinte erro:
Undefined index: datahora_final

Answer 1

If the final date field is being filled with date in the format 20/05/2017 then the problem is no date('Y-m-d H:i:s', $datahora_final) do so:

$datahora_final_sem_formatar   = $_POST['datahora_final'];
$datahora_final   = substr($datahora_final_sem_formatar,6,4) . "-" . substr($datahora_final_sem_formatar,3,2) . "-" . substr($datahora_final_sem_formatar,0,2);